drm/amdgpu: drop amdgpu_gtt_node

We have the BO pointer in the base structure now as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321132601.2161-6-christian.koenig@amd.com